Throw more user-friendly exception when we can't find a runner.
Throw more user-friendly exception when we can't find a runner.

Plexxi/st2,nzlosh/st2,Plexxi/st2,StackStorm/st2,nzlosh/st2,Plexxi/st2,StackStorm/st2,Plexxi/st2,nzlosh/st2,StackStorm/st2,StackStorm/st2,nzlosh/st2